This invention relates generally to techniques for fine feature lithography. More particularly, it relates to a method of enhancing resolution. Even more particularly, it relates to a method of compensating for mechanisms that degrade printed image quality. Finally it relates to a method of providing improved fidelity in providing patterns used in integrated circuits.
There is a continuing trend in the field of integrated circuits to reduce the size of the circuitry. As a result, there is an increased demand to improve the resolution of projection printers and other devices used in lithography during the manufacture of integrated circuits.
FIG. 14 illustrates a mask pattern 10 to be formed on a substrate 12. FIG. 15 is an illustration of a printed image 14 formed in a photoresist using the mask pattern 10 shown in FIG. 14. As is shown in FIG. 15, the long axis LA2 of the printed image 14 shortens as compared to the long axis LA1 of mask pattern 10. This will henceforth be referred to as foreshortening.
FIG. 16 illustrates square mask image 190 to be formed on a substrate 12. FIG. 17 is an illustration of the printed image 190xe2x80x2 using the mask image 190 shown in FIG. 16. As shown in FIG. 17, the comers of the image 190xe2x80x2 become round during printing and developing compared to the mask image 190, because of optical (e.g., diffraction) and resist processing effects. This process is referred to here as xe2x80x9ccorner rounding.xe2x80x9d
FIG. 18 illustrates another mask pattern 40 containing both nested features 42a, 42b and an isolated feature 44. FIG. 19 is an illustration of printed image 40xe2x80x2 using mask pattern 40 shown in FIG. 18. As demonstrated in FIG. 19, isolated feature 44xe2x80x2 prints differently than nested features 42axe2x80x2 and 42bxe2x80x2. (Depending on effects such as diffraction, lens aberrations, and resist characteristics, isolated feature 44xe2x80x2 can print either larger or smaller than nested feature 42axe2x80x2 and 42bxe2x80x2.) Also, the width of outside images 42bxe2x80x2 in a group of nested images print differently than inside images 42axe2x80x2. These two effects are referred to here as xe2x80x9cnested-to-isolated print bias.xe2x80x9d Outside images 42bxe2x80x2 are nested only on one side; consequently they experience less of a proximity effect than fully nested images 42axe2x80x2. 
FIG. 20 illustrates a mask which includes bar 50 and contact 52. FIG. 21 shows printed bar image 50xe2x80x2 associated with mask bar image 50 and printed contact image 52xe2x80x2 associated with mask image 52. When the bar image 50xe2x80x2 is printed optimally, contact image 52xe2x80x2 is poorly imaged, as shown in FIG. 21. If contact image 52xe2x80x2 is printed optimally, bar image 50xe2x80x2 is printed too wide. This effect is referenced to here as xe2x80x9cfeature size dependent bias.xe2x80x9d
Adjustments of the mask image to compensate for the foreshortening may be difficult when small images are to be formed in the photoresist. There is limited space between the images in arrays of high-density small images to apply compensation at the mask level (i.e., to grow the image in the direction that it will foreshorten) without introducing undesirable proximity effects such as image bridging.
One prior art technique to improve resolution is to use contrast enhanced layers (CEL) with projection printers. A photo-bleachable layer is spun onto a photoresist-coated wafer to a thickness of approximately 1,000-3,000 xc3x85. Upon exposure of the CEL by the aerial image produced by the mask and projection printer, the regions of the photo-bleachable layer that are exposed to the highest intensities bleach through first, while those areas of the photo-bleachable layer that receive the lowest intensity bleach through at a later time. Therefore, windows in the CEL are formed which allow the imaging light to pass through and expose the underlying photoresist. The net effect is to increase the effective contrast of the CEL and photoresist stack over that of the photoresist alone. Contrast enhanced imaging or multilayer resists alone may be inadequate, however, for significant improvement of image foreshortening in advanced lithography.
To overcome the shortcomings of the prior art, a new process is provided. An object of the present invention is to provide an improved process to reduce image foreshortening, corner rounding, nested to isolated print bias, and other print biases.
To achieve this and other objects, and in view of its purposes, the present invention provides a process of forming a first device pattern having a first end, a second end, and a center area. The process comprises the steps of providing a substrate having a photosensitive coating, forming the center area of the first device pattern on the photosensitive coating, and forming one of the first end and the second end of the first device pattern on the photosensitive coating.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.